It’s a rule in my class that our classroom before 6:00 p.m. every day.A. be able to clean

问题补充：

It’s a rule in my class that our classroom before 6:00 p.m. every day.

A. be able to clean

B. should be cleaning

C. must clean

D. must be cleaned

答案：

A. be able to clean

B. should be cleaning

C. must clean

(答案→)D. must be cleaned

解析：be able to能，强调自身能力能够做某事；should应该；must一定。句意：每天下午六点前必须打扫教室，在我们班是一条规定。结合语境可知从句主语是动作的对象，故用被动语态，选D。